Frankia Symbiosis

The 12th meeting on Frankia and Actinorhizal Plants that took place in Carry-le-Rouet, France in June 2001 was the opportunity for scientists to communicate about latest developments on this symbiosis that concerns a wide range of dicotyledonous plants, initiates ecological successions and is used in a number of countries to protect crops from winds or improve soil status. Selected papers on plant ecology, Frankia's genetics or physiology, and host plants' genetics or physiology are published in this special Plant & Soil issue.

The Biology of Frankia and Actinorhizal Plants provides a comprehensive review of Frankia and the actinorhizal plants. It reviews the state of knowledge on all aspects from molecular genetics through ecology to practical applications; describes methods used in research and practical applications; and is a guide to the literature. The book begins with overviews of Frankia and the actinorhizal plants, and developments in the field prior to the first confirmed isolation of Frankia. Next is a series of authoritative chapters on the biology of Frankia, the symbiosis, and actinorhizal plants. Although methods used in research and in practical applications are included throughout the book, they are given special emphasis in the middle section. The final section of the book concerns the ecology and current and potential uses of actinorhizal plants in both the temperate regions and the tropics. This work is intended as a reference text and handbook of methods for a wide audience including established workers and students of Frankia and actinorhizal plants, specialists and students in other areas of nitrogen fixation (including the Rhizobium-legume symbiosis), soil microbiologists, plant physiologists, ecologists, general biologists, foresters, specialists in land reclamation, and managers requiring an authoritative overview of this rapidly developing field.

The fifth meeting of scientists working with Frankia and actinorhizal plants was held at Montmorency Forest of Laval University in Quebec from August 6-8, 1984. Results of research presented at the meeting are included in this special volume of Plant and Soil. The understanding of actinorhizal systems continue to increase, though work and use shops and discussions at this and similar meetings make it evident that this important subject remains open for fruitful investigation at all levels. Some important 'firsts' were reported at this meeting. The first extensive survey of Frankiae and their host specificity ranges from Asia was presented. This is of significance since Asia is a center of diversity for many actinorhizal host plant genera. A report that proto plasts of Frankia have been produced and regenerated for the first time improves the possibility for genetic manipulation of Frankia. It is also important to note the first report herein of successful mass inoculation of actinorhizal plants commercially for stabilization and reclamation of disturbed soils around hydroelectric power projects in Quebec. This heralds the transfer of actinorhizal technology to private and public users. The bacterial genus Frankia is easily recognized both in vivo and in vitro, and isolation of this organism has become routine. But, as yet, there are not sufficient biochemical, morphological, or anatomical criteria for establishing species.

Five years have now passed since the first symposium on frankiae was held at Harvard Forest, Petersham, Massachusetts, USA and the inauguration of the term actinorhiza. Many advances have been made during these five years in our understanding of the actinorhizal symbioses. Evidence for this was provided by the papers presented at the Wageningen Workshop on Frankia symbioses, held in Wageningen at the Department of Microbiology of the Agricultural University. Most of these papers are now published in this volume of PLANT AND SOIL. We kindly acknowledge the assistance of Anton Houwers, editor of the journal, in planning, reviewing and publishing these studies. Although the papers presented at Wageningen described the active research areas, they also illuminated those aspects of these symbioses which remain beyond our understanding. Primary among the areas of our ignorance is the concept of species within the bacterial symbiont, Frankia. At present groupings of bacterial strains are based on cell chemistry, physiology, serology, DNA homology and symbiotic capa bilities (cross-inoculation). When these classification schemes are merged no clear species framework is obtained. Undoubtedly part of the difficulty is due to a lack of strains for analysis. Currently bacterial strains from only half of the actinorhizal symbioses known to exist, have been isolated and studied in pure culture. We must postpone there fore any comprehensive taxonomic classification until a larger majority of the symbioses are represented. Another research area wherein our understanding is insufficient is host-symbiont interaction.

Nitrogen availability is one of the most critical factors that limits plant productivity. The largest reservoir of nitrogen is the atmosphere, but this gaseous molecular nitrogen only becomes available to plants through the biological nitrogen fixation process, which only prokaryotic cells have developed. The discovery that microbes were providing fixed nitrogen to legumes and the isolation of the first nitrogen-fixing bacteria occured at the end the 19th Century, in Louis Pasteur's time. We are now building on more than 100 years of research in this field and looking towards the 21st Century. The International Nitrogen Fixation Congress series Started more than 20 years ago. The format of this Congress is designed to gather scientists from very diverse origins, backgrounds, interests and scientific approaches and is a forum where fundamental knowledge is discussed alongside applied research. This confluence of perspectives is, we believe, extremely beneficial in raising new ideas, questions and concepts.

In the 100 years since the legume-Rhizobium symbiotic nitrogen fixation interaction was first described, interest in this field has grown rapidly. The types of studies have been cyclical in nature, involving a cross-section of disciplines. The availability of cheap nitrogenous fertilizers caused much of the biological nitrogen fixation research to become more theoretical in the developed world. The high cost of energy, coupled with environmental concerns and the interest in sustainable agriculture, has stimulated research in symbiotic nitrogen fixation. The development of modern genetic techniques has resulted in interdisciplinary research on plant-microbe interactions controlling nitrogen fixation. This has resulted in a better understanding of environmental factors influencing the nodulation process, chemical signalling between the symbiotic partners and the nature of the specificity between host plant and microsymbiotant. This volume summarizes the diverse research efforts in biological nitrogen fixation by presenting a collection of papers in the areas of physiology and metabolism, taxonomy and evolution, genetics and ecology.

A multiplicity of biotrophic micro-organisms interact with plants in nature, forming symbiotic relationships that range from mutualism to antagonism. Microorganisms that have adopted biotrophy as a lifestyle are able to colonize the plant and often to cross the plant cell boundaries by forming intracellular structures that are the site of nutrient uptake/exchange. To establish themselves within plant tissues, both mutualistic and pathogenic biotrophs need to overcome the plant defense response through an exchange of molecular signals. Our knowledge of the nature of these signals and their function in the interaction has rapidly increased over the last few years. This volume focuses on the genetic, molecular and cellular components involved in the communication between partners of well-known symbioses, but also reports on the advances for less studied systems.

The core of the text is aimed at the research worker in the field of nitrogen fixation, but, despite its specialisation, does not lose the emphasis on teaching, both as a direct reference book and as a backbone for a graduate course on the subject.The closing part of the book includes a subject index and a glossary of terms. The latter was included not for the expert, for whom many of the definitions will be too general, but for the newcomer; the author hopes that the quick survey of key terms will help in the reading of this book.